This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This description is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Most raw natural gas extracted from the Earth contains primarily methane (CH4) and also contains, to varying degrees, low and high molecular weight hydrocarbon compounds. The primary component methane (CH4), as a low molecular weight hydrocarbon, is typically a desirable component within harvested natural gas. Today, purified CH4 is viewed as a valuable energy source because it is generally considered as a clean-burning fuel in numerous applications. Compared to other hydrocarbon fuels, the burning of CH4 produces less carbon dioxide (CO2) emissions for each unit of heat released. Additionally, based on its ratio of heat of combustion to its molecular mass, CH4 produces more heat per mass unit than complex hydrocarbons. Furthermore, CH4 may generally be transported with ease. Thus, in many cities, CH4 is piped into homes for domestic heating and cooking purposes as an efficient fuel. In this context, CH4 is usually known as natural gas, which has an energy content of ˜1,000 BTU per standard cubic foot in certain examples. In the form of compressed natural gas, CH4 may be used as a vehicle fuel where it may be more environmentally friendly than other fossil fuels such as gasoline or diesel.
Raw natural gas may need to be processed to remove contaminants and impurities such as heavier hydrocarbons including ethane (C2H6), propane (C3H8), and butane (C4H10), among others. When brought to the surface and processed along with the CH4, such heavier hydrocarbons are collectively referred to as Natural Gas Liquids (NGLs). The raw natural gas may also include acid gas contaminants such as carbon dioxide (CO2) and hydrogen sulfide (H2S), and mercaptans, such as methanethiol (CH3SH) and ethanethiol (C2H5SH). Additionally, the raw natural gas may contain contaminants including nitrogen (N2), helium (He), water vapor, liquid water, mercury, and natural gas condensate.
The heavier hydrocarbons, NGLs, and contaminants within the raw natural gas may lead to equipment malfunction, production failure, product contamination, among other detrimental production issues. For example, when the acid gas contaminant CO2 is combined with water, it may create a corrosive form of carbonic acid. Additionally, CO2 will reduce the BTU value of the natural gas and lower the economic viability of the natural gas, for example, in concentrations of more than 2%. Similarly, H2S can dissolve in water to create a highly corrosive acid that can attack metal structures. Moreover, water in the form of a vapor or liquid within a raw natural gas may form hydrates, thus, potentially leading to plugging of pipelines. Thus, it may be economically beneficial to remove the contaminants from the natural gas to produce purified CH4.
The separation techniques for purifying raw natural gas may utilize flash drums, separators, and distillation and fractionation towers. In some cases, the separation techniques may embody cryogenic temperatures where CO2 may solidify and fall out of the natural gas. Other technologies for the removal of CO2 from natural gas are based on principles that do not involve cryogenic temperatures. For example, some techniques may be solvent-based, such as capturing CO2 with a chemical, physical, or hybrid solvent, and reversing the process to remove the captured CO2.
U.S. Pat. No. 7,325,415 discloses a process and device for the removal of solid freezable species such as carbon dioxide, water, and heavy hydrocarbons from a natural gas feed stream during liquefaction to produce LNG. The solid freezable species may be removed on a continuous basis following liquefaction of the natural gas feed stream. The solid freezable species may then be liquefied on a continuous basis if required. Continuous removal of the freezable species from the natural gas feed stream is apparently achieved by maintaining both cooling and separation apparatuses at the same working pressure. The technique provides that at least part of the cooling vessel is constructed from a material having a low thermal conductivity which discourages formation of the solids of the freezable species on the walls of the cooling vessel.
U.S. Pat. No. 6,755,965 discloses a process for ethane extraction from a gas stream based on turbo-expansion and fractionation with no mechanical refrigeration. The feed gas is sweetened and dehydrated by a conventional amine process followed by a molecular sieve unit to remove carbon dioxide and water. After this pretreatment, the feed gas undergoes a series of cooling steps through a cryogenic brazed aluminum heat exchanger and is fed to a de-methanizer column. A rich-methane stream is recovered from the top of this column and fed to a centrifugal compressor and subsequently routed to a booster/turbo-expander. The temperature of the methane gas is reduced by the expansion allowing the cooled methane stream to be a cooling source for the cryogenic heat exchanger. A feed for a de-ethanizer column comes from the bottom liquids of the de-methanizer column. Thus, ethane is recovered overhead from the de-ethanizer column.
U.S. Pat. No. 6,516,631 discloses a cryogenic natural gas liquids recovery process, which includes the use of a de-methanizer and a de-ethanizer. The recovery process also includes a step of recycling a portion of the de-ethanizer overhead to the de-methanizer.
U.S. Pat. No. 6,082,133 discloses an apparatus for separating CO2 from a mixture of gases having CO2 and a second gas, where the apparatus includes an active heat exchanger and a regenerating heat exchanger. The mixture of gases is present in the active heat exchanger at a predetermined pressure, which is chosen such that CO2 freezes on the heat exchanger surface. The heat exchanger surface is cooled by a refrigerant having a temperature below that at which CO2 freezes at the predetermined pressure. The regenerating heat exchanger includes a heat exchange surface in contact with the refrigerant and also in contact with a layer of frozen CO2. The refrigerant enters the regenerating heat exchanger at a temperature above that at which the CO2 in the frozen layer of CO2 sublimates. The sublimation of the solid CO2 cools the refrigerant prior to the refrigerant being expanded through an expansion valve, which reduces the temperature of the refrigerant to a point below the freezing point of CO2 at the predetermined pressure. The refrigerant is re-compressed by a compressor after leaving the active heat exchanger. A second precooling heat exchanger precools the compressed refrigerant by providing thermal contact with the refrigerant leaving the active heat exchanger.
U.S. Pat. No. 5,819,555 discloses a process to remove CO2 from a feed stream. The solid forming property of CO2 and the low vapor phase solubility of carbon dioxide at cold temperatures form the basis for the separation process. The cooled feed stream enters a separation vessel where process means are provided to produce and separate CO2 solids. The CO2 is removed from the vessel as a CO2 rich liquid stream, and a purified cold vapor is removed from the separation vessel as a product stream.
The aforementioned techniques may provide for purifying a raw gas stream. However, there remains an ongoing need for more efficient separation techniques to purify the raw gas stream by removing CO2 to produce purified CH4 for use as a valuable energy source.